The present invention relates to a cutting device for creating a slit opening for a buttonhole.
A cutting device of this kind in a buttonhole sewing machine is known from a brochure entitled DURKOPP ADLER AG, 576, imprint 0002 009122 2470 D/GB 12/97. With it, a slit opening for a buttonhole can be created in two ways, in a slit-cutting mode or in a block-cutting mode. Two tool parts are used for this process.
In the slit-cutting mode, the slit opening is created by a knife that enters into a slit in a supporting plate on which lies the workpiece. In the block-cutting mode, in contrast, the slit opening is created by placing a knife on a flat working surface of a so-called cutting block, whereby the knife penetrates the workpiece placed on the working surface and thereby creates the slit opening. One cutting mode or the other is used according to the material properties of each workpiece to be cut.
In the known cutting device, conversion of the working mode from one cutting mode to the other is carried out by exchanging the tool parts, that is, by using associated tool parts for each mode. While the moving tool parts can be exchanged without problems, changing the cutting or supporting surfaces involves a significant expenditure in cost and time. This is because of the fact that a bearing plate consisting of a large number of parts, including a thread-cutting device, must be exchanged. Accordingly, in order to implement one cutting mode or the other, it is necessary to store the tool parts required for both modes. If, on the other hand, a customer's buttonhole sewing machine equipped for one cutting mode or the other is to be converted, conversion is possible only if the corresponding tool parts are present or purchased, which involves a certain delivery time and cost for the tool parts.
Another disadvantage of the known cutting device is the difficulty of securing and removing the cutting block to and from the slit in the bearing plate. The slit and the cutting block are dimensioned in such a way that the cutting block must be pressed manually into the slit with great force and also must be pressed out manually with great force. For this, the cutting block is constructed with chamfers, by means of which intermediate spaces are formed at the sides of the cutting block. For this reason, only one good clamping position of the workpiece is possible, which affects the precision of the placement of the cut between the stitch rows.
With this type of attachment, i.e., with the chamfers provided on the cutting block, it is not possible to hold the cutting block on its sides. This presents an especially critical problem when the cutting block--as is the case in the known cutting device--is made of plastic and the danger exists of premature splitting because of the penetrating knife.
The plastic block accepted by the bearing plate is constructed with a free cut at each of its ends, so that reuse of a cutting block that has been split on one side is limited to using one other position, i.e. there are only two usable positions. Accordingly, it is necessary to provide sufficient spare cutting blocks.